The present invention relates to a semiconductor device comprising an insulating or conducting substrate and a GaN-based compound semiconductor layer or a ZnSe-based II--VI compound semiconductor layer formed on the substrate. More specifically, the present invention relates to a semiconductor device whose operating voltage is much reduced by providing a material playing a part in reducing the resistance, between a compound semiconductor layer and a growth substrate, between the compound semiconductor layer and a metal electrode, between the growth substrate and the metal electrode, or between individual compound semiconductor layers. The present invention further relates to a method of producing the semiconductor device.
The GaN-based compound semiconductor device hitherto-most widely employed has an electrode structure called "side injection structure". The side injection structure is formed by growing the GaN-based compound layer on a single crystalline insulating substrate such as sapphire, subjecting the resultant structure to mesa formation, providing a first electrode for current supply on a top of the mesa, and extending a second electrode for current supply from the lower part of the mesa along the upper surface of the insulating substrate. Such a structure is also employed in a semiconductor device comprising a ZnSe-based compound.
Compared to a semiconductor device having a first electrode on the top of the semiconductor layers grown on a conductive substrate and a second electrode on the lower side of the conductive substrate, the semiconductor device having the side injection electrode structure has a drawback in that it tends to have a high resistance ascribed to its structure.
Hereinafter, a laser diode will be referred to as LD, light emitting diode as LED, the first electrode as upper electrode, and the second electrode as lower electrode. Suffixes representing the compositions of mixed crystal compound semiconductors will be omitted unless otherwise needed.
If the contact region of electrodes of the semiconductor substrate exhibits a high resistance, the operating voltage of the semiconductor device will increase. Consequently, the device will fail to operate continuously for a long time. Furthermore, the high resistance influences the current distribution density within the device, providing an adverse effect on the operation performance of the device.
FIG. 1 is a cross-sectional view schematically showing the current density distribution within a conventional GaN-based LD grown on a sapphire substrate. Curved lines drawn on the cross-section of the multiple-layer structure indicate the profile of the current density distribution. A GaN-based LD or a GaN-based LED emitting blue to violet has an active layer 7 made of InGaN. On the upper and low sides of the active layer 7, a lower optical guide layer 6 made of n-type GaN (n-GaN) and an upper optical guide layer 8 made of p-type GaN (p-GaN) are respectively provided. Adjacent to the optical guide layers 6 and 8, a cladding layer 5 made of n-AlGaN and a cladding layer 9 made of p-AlGaN for optical confinement are respectively formed.
On the upper and the lower sides of this double-hetero structure, which is a standard LD structure, a lower contact layer 4, made of n-GaN, and an upper contact layer 10, made of p-GaN, are respectively provided, in order to reduce the ohmic resistance of the electrodes. The n-GaN contact layer 4, first grown on the sapphire substrate, is laterally extended and a lower electrode 3 for operating-current supply is formed on the extended part of the n-GaN contact layer 4. In FIG. 1, an SiO.sub.2 film, 11, which is formed by etching in the form of stripe, constricts the current supplied to the active layer 7 into the area under the aforementioned stripe. Thus, the SiO.sub.2 film serves as a current blocking insulator film for reducing the operating current. The n-GaN contact layer 4 has a sheet resistance in the lateral direction, so that a potential drop occurs in the lateral direction to the n-GaN contact layer 4 serving as a cathode of the device.
It is desirable that the current density distribution be longitudinally uniform between the anode and the cathode of the device. However, in the device (FIG. 1), which is characterized by the side injection structure, consisting of an insulating substrate and multiple-layer compound semiconductor formed thereon, the passage of the operating current is significantly skewed toward the mesa edge on which the side injection electrode 3 is provided.
The skewed current-path has an effect on the light emission pattern, as shown in FIG. 2. The lengthwise width of the active layer 7 shown in FIG. 1 is plotted on the abscissa. Plotted on the ordinate is the output light intensity which is measured near the aperture of the active layer 7.
As is indicated by a solid line in FIG. 2, a skewed current density distribution, due to the skewed current-path described above, results in an nonuniform distribution of the output light intensity and adversely affects LD performance.
In particular, in the case where the GaN-based multiple-layer structure is constructed in the inverse order from that in FIG. 1, by growing the p-GaN layer first on a sapphire substrate and providing a side injection electrode on the first-grown p-GaN layer, due to the hole mobility being so low, the above effect is even stronger. Hence, a LD or LED having such a structure tends to have a high operating voltage and the current density distribution is skewed even more.
Accordingly, to attain a desired LD or LED working at a low voltage, it may be better to use a conductive substrate such as SiC, since it lattice matches with the GaN-based compound.
However, SiC is expensive and it is impossible to obtain a single crystalline SiC having a large enough diameter required for a large-scale production. When GaN layer is grown onto the SiC substrate, no barrier for electron current is produced in theory. However, in practice, the interface between the GaN layer and the SiC substrate tends to become highly-resistant to the electron current flow. This highly-resistant interface layer results from an adverse interface reaction between the SiC substrate and the GaN layer.
For this reason, an inexpensive sapphire substrate has been used heretofore in the GaN-based semiconductor devices. However, 16% of a lattice mismatch is present between the GaN layer and the sapphire substrate, producing a large lattice strain in the GaN growth layers. These lattice strains develop into lattice defects in the growth layer. This is one of the reasons why it has been difficult to manufacture practical GaN-based LD's and LED's.
Since sapphire is an insulating substrate, a side injection electrode for current supply has to be provided on the first-formed n-GaN layer on the sapphire substrate. Even if a relatively-low-resistant n-GaN layer is used as the side injection layer, the resultant LD and LED formed on the sapphire substrate and provided with the side injection structure will have a large device resistance, compared to the case where the conductive substrate is used. As a result, the operating voltage is increased.
As described above, problems are present not only in the lower electrode but also in the upper electrode. Due to the high operating current density, metallic atoms of small-atomic radii forming the electrode easily migrate into the GaN layer or into a mixed crystal, degrading the working function of the semiconductor device. The problem of electromigration is not always caused by metallic atoms having small-atomic radii alone. In an attempt to prevent this electromigration, a metallic material having large atomic radii, like a barrier metal, was employed as an electrode instead of metallic atoms having small-atomic radii. However, this attempt was unsuccessful. Hence, electromigration still remains a difficult problem to be solved.
The GaN-based material has a drawback in that its dislocation density is high, of the order of 10.sup.8 cm.sup.-2, and has a tendency to propagate particularly in the direction of crystal growth. As shown in FIGS. 3A and 3B, when Au is used, for example, as an electrode material, Au ions are deposited along the dislocation line via electromigration caused by the flow of electric current. When the Au deposition gets deep enough into the crystal and dense enough, short-circuit of the GaN based multilayered structure occurs, causing a catastrophic failure of the semiconductor device. FIG. 4 is a schematic cross-sectional view of a GaN-based LD having a failure due to a short. The profile of the Au ions transported by the current can be microscopically observed.
As is mentioned above, the conventional semiconductor device having the side injection structure has two problems. One is the generation of excessive operating voltage due to high series resistance. The other is the migration of metal atoms from the upper electrode into semiconductor layers due to highly-dense operating current and the high dislocation density.